Characterization of Electrodes and Electrolytes for Aqueous Organic Redox Flow Batteries Using Static Cells

Abstract

Energy storage systems have become an essential component to the renewable energy transition. Aqueous organic redox flow batteries (AORFBs), specifically, have garnered interest as stationary battery technologies to solve the issue of intermittency of renewable energy for grid-scale electricity generation. In this work, we introduce a simplified, static cell design to enable straightforward evaluation of extremely low capacity fade rates for flow battery electrolytes using battery cycling methods. These static cells demonstrate the capability to reduce standard deviations in measured overall capacity fade rates per day across multiple experiments compared to measurements in flow cells. Furthermore, they allow the capability to, for the first time, decouple contributions to capacity fade due to time- and cycling rate-denominated fade because of their small volumes. Using porous electrode theory, we simulate the physics of reactions and transport inside the cell to investigate the roles of imperfect impregnation of the electrodes in the cell by the electrolyte. Then, using variations of the original static cell design, we investigate the confinement of organic molecules in micropores with surface characterization and voltammetry. Macroscopic and microscopic simulations confirm micropore confinement can influence the thermodynamics of charge transfer. We investigate the performance of several organic molecules in microporous electrodes and elucidate trends between electrophilicity, charge, and molecular structure on activity in the micropore. Finally, we demonstrate the ability to increase the energy density of aqueous organic secondary batteries in the form of static cells and flow cells by using micropore confinement of the active species.